Because the carbonate springs have an excellent heat retaining effect, the carbonate springs is used in a bath house and the like in which a hot spring is utilized from a long time ago. Basically the heat retaining effect of the carbonate springs is considered as improvement of a body situation by an angiotelectasia effect of a contained carbonic acid gas. It is also considered that an increase and expansion of a capillary bed occur by intrusion of the carbonic acid gas into a skin to improve skin blood circulation. Therefore, it is regarded that the carbonate springs are effective in treating a regressive change and a peripheral circulatory disorder.
Recently, in the treatment of the regressive change and the peripheral circulatory disorder, it is found that a physiological effect of the carbonate springs can further remarkably be exerted when a carbon dioxide concentration in the carbonate springs becomes about 1200 mg/L (liter) which is a supersaturated concentration range in hot water having a temperature of about 40° C.
Examples of method of synthetically producing the carbonate springs include a carbonate spring producing method of circulating the hot water in the bath through a carbonic acid gas dissolver with a circulating pump in a circulation type carbonate spring producing system and a carbonate spring producing method of producing carbonate hot water by passing the hot water supplied from a water heater or the like through the carbonic acid gas dissolver once with one-pass type carbonate spring producing system. For example, a static mixer and a hollow fiber membrane module are often used as the carbonic acid gas dissolver having good dissolution efficiency.
However, even if such carbonic acid gas dissolvers are used, the carbonic acid gas cannot be dissolved in the hot water at the concentration of 100%. In this case, un-dissolved carbonic acid gas is wastefully emitted in the atmosphere, which generates a large problem from the viewpoint of running cost. The un-dissolved carbonic acid gas mixed in a bubble in the carbonate springs is emitted into a bath room, and the bath room is in the high concentration atmosphere of the carbonic acid gas in the case where a large amount of carbonate springs is produced like full immersion bath, which possibly has an adverse affect on a human body.
A threshold limit value (TLV) of the carbonic acid gas concentration is not more than 0.5% in a room. When the carbonic acid gas concentration becomes not lower than 10%, adjustment functions of the human body are disabled, and a person becomes unconscious in about ten minutes. When the carbonic acid gas concentration becomes not lower than 25%, it is said that respiration becomes slow and a person dies in several hours (for example, see Non-Patent Document 1).
For example, there is proposed a carbonate spring producing system, in which the un-dissolved carbonic acid gas separated by a gas separator is recovered by introducing the un-dissolved carbonic acid gas to a compressor and the recovered carbonic acid gas is introduced to the carbonic acid gas dissolver to dissolve the carbonic acid gas in the hot water (for example, see Patent Document 1).
In the carbonate spring producing system described in Patent Document 1, the un-dissolved carbonic acid gas separated by the gas separator is recovered with the compressor, and the recovered carbonic acid gas is sent to the carbonic acid gas dissolver again and utilized to produce the carbonate springs. The inventors have been proposed the carbonate spring producing system described in Patent Document 1.
A carbonic acid gas neutralization apparatus is proposed as an example in which the carbonic acid gas is dissolved in the liquid (for example, see Patent Document 2). In the carbonic acid gas neutralization apparatus described in Patent Document 2, the un-dissolved carbonic acid gas separated by gas-liquid separating means is injected in an upstream of a pump which sends the alkaline drain solution to mix the un-dissolved carbonic acid gas with the hot water, or the un-dissolved carbonic acid gas is mixed in the hot water such that an ejector in which the alkaline drain solution is utilized as a driving solution is used as a gas injection nozzle to suck the un-dissolved carbonic acid gas from the ejector.
Examples of a method of measuring the carbonic acid gas concentration in the carbonate springs includes a method in which an ion-electrode type carbonic acid gas concentration meter is used, a method of computing the concentration from a pH measurement value with a pH meter (for example, see Patent Document 3), and a method in which an amount of bubble existing in the carbonate springs is measured with an ultrasonic wave sensor to compute the concentration from the measured bubble amount (for example, see Patent Document 4). The inventors have been proposed the methods of measuring the carbonic acid gas concentration in the carbonate springs described in Patent Documents 3 and 4.
Patent Document 1: Japanese Patent Application Laid-Open No. H11-192421
Patent Document 2: Japanese Patent Application Laid-Open No. 2001-170659
Patent Document 3: Japanese Patent Application Laid-Open No. 2003-066023
Patent Document 4: International Patent Publication No. WO 2003/020405
Non-Patent Document 1: Security (Iwatani High-pressure Gas Security Information Journal, Vol. 63 (2003)